Quarto rendering error - volume doesn't contain a recognised file system

I'm getting the following error when trying to render a quarto markdown document and not sure how to debug.

==> quarto preview README.qmd --to gfm --no-watch-inputs --no-browse

ERROR: The volume does not contain a recognized file system.
Please make sure that all required file system drivers are loaded and that the volume is not corrupted. (os error 1005): stat 'README.qmd'

Stack trace:
Please make sure that all required file system drivers are loaded and that the volume is not corrupted. (os error 1005): stat 'README.qmd'
    at Object.statSync (ext:deno_fs/30_fs.js:363:7)
    at existsSync (file:///C:/PROGRA~1/RStudio/RESOUR~1/app/bin/quarto/bin/quarto.js:495:27)
    at Command.fn (file:///C:/PROGRA~1/RStudio/RESOUR~1/app/bin/quarto/bin/quarto.js:97989:10)
    at async Command.execute (file:///C:/PROGRA~1/RStudio/RESOUR~1/app/bin/quarto/bin/quarto.js:8104:13)
    at async quarto (file:///C:/PROGRA~1/RStudio/RESOUR~1/app/bin/quarto/bin/quarto.js:115002:5)
    at async file:///C:/PROGRA~1/RStudio/RESOUR~1/app/bin/quarto/bin/quarto.js:115020:9

It was working fine, till I tried to change my header in the qmd document to:

title: "My title"
format: 
  gfm:
    output-file: "README"

from what it originally was (when working):

title: "My title"
format: gfm

Now even though I change it back it doesn't work.

Any ideas on what has happened, and what I can do will be appreciated!